1. Field of the Invention
The present invention relates to a railway car retarder for shunting purposes consisting of the type of automatic retarder units mounted one after the other along a railway track, each unit being provided with an actuating member, which when contacted by wheels of passing railway cars will exert a retarding effect on the wheels provided the speed of said wheels exceeds an adjustable value pre-set for each retarder unit.
2. Description of the Prior Art
Braking units of this type are known in several structurally different embodiments. A common feature of the embodiments which in recent years have been increasingly used is that they are of a hydraulic type, by which the passing wheel via a pedal, a ramp or the like is urged to pump a hydraulic liquid in a closed circuit in the braking unit. The liquid passes through a flow-depending throttling device, which means that the work will be large at high-flow speeds and negligibly small at low-flow speeds.
Hydraulic systems with the energy absorption here concerned with require high quality material and precision manufacture whereby they will also be expensive. It is also difficult to make, e.g., inlets for shafts and piston rods so that during several years' use and in a severe working environment they will be leakproof. The demands for leakproofness and cleanness furthermore mean that inspection and adjustment of the inner components of the brake cannot be accomplished in the field but must take place in special workshops by particularly trained personnel. The hydraulic brakes furthermore produce a certain unintended braking effect even when the cars pass at such a low speed that it is desired that they are not subjected to any braking effect whatsoever.
These drawbacks, among others, make it desirable to develop a retarder unit with the same speed-dependent working characteristics as mentioned above without utilizing any hydraulic system for the energy absorption, e.g., in the form of a simple mechanical friction brake. However, development in this direction has been hampered by the fact that the brake force must be so accurately balanced that a maximum braking effect is achieved without the risk that the brake will lift a light car wheel from the track. A hydraulic pressure can be adjusted very accurately whereas friction coefficient variations at, e.g., a disc brake can give considerable variations in the braking ability due to temperature and moisture variations, material wear, etc. A well-designed hydraulic brake has consequently a long life span and mainly constant braking characteristics, as this brake will get adequate lubrication and is subjected only to a minimum of wear as the braking energy is transferred into heat due to flow losses in the liquid and not by mechanical friction. In mechanical brakes it is, on the contrary, inevitable that the friction surfaces are subjected to a certain wear which necessitates a continuous readjustment of the positions of the braking surfaces concurrently with the wear. Finally, it is rather simple to design a hydraulic brake so that the flow speed of the hydraulic liquid constitutes a representative measure of the speed of the car wheel passing the brake, and it is no problem to provide the system with a flow-depending valve, which controls the relation between the car speed and the throttling effect. In disc brakes or similar mechanical brakes, there is, on the contrary, no real relationship between speed and braking torque, but the braking torque is quite independent of the speed.
Thus it can be said that small hydraulic brakes having three essential basic properties are well known. These three properties are: (1) a built-in speed metering which controls the braking action; (2) a well-defined braking power which allows a heavy braking without exceeding the limit at which light cars are raised from the track; and (3) that the properties will remain unaltered for a long period of time, whereby surveillance and wear adjustment are seldom required.
These three basic properties are obvious and easy to obtain in hydraulic systems, but such brakes are expensive for railway marshalling purposes. Mechanical brakes would be much less expensive but they lack the natural prerequisites of having said three properties.